(...) So, let's cut the story-telling (sorry Dreambeard, although I really love your contributions, they are not helping here) ...

Sorry for behaving like a young hobbit. I saw that phrase in your post, and because it was exactly what I had just been pondering about, I reacted in a kind of reflex. But of course it had very liitle bearing on the subject, so I should have posted it elsewhere.

The actual theme of your post is something that I have been thinking about too. Not with a rolling ball, but with something similar to what Sky suggested, some wand of blasting that would be useful to blow open doors, trigger trapsdoors and pits and some other traps.
Of course, summoning traps are rune traps, but I always thought of these as a combination of a mechanical (weight/pressure) trigger linked to the runic spell. I find it hard to make sense of a runic trigger. After all, you make the trap go off by stepping on the tile; not by reading the rune. So I also thought of heavy items that you could perhaps throw onto a trapped tile to make the trap go off.
Not balls - there are no cannons in angband - but chests or heavy body armor. But the problem with those is that you cannot roll them onto the trap, and you cannot throw them very far either (I think). So you may still be surrounded by opponents when the summoning trap goes off.

Yet another option would be that some monster could make the trap go off.
The rule in angband is that monsters do not trigger traps, "because they know exactly where they are". But what about a confused monster, then? Or a terrified monster?
Personally I think that that would be the most plausible way to let a summoning trap go off with you at some distance: lure a human(oid) monster - or better still, a group - over there, confuse the lot (with a staff), blink out of the way, and cross your thumbs that one of them will hit the trap.